1. Field of the Invention
The present invention relates to dual-band oscillators which contain two oscillation circuits and which oscillate in different frequency bands.
2. Description of the Related Art
FIG. 3 is a circuit diagram of a known dual-band oscillator which oscillates in two frequency bands. A first oscillation circuit 41 contains a first common-collector oscillation transistor 42. The emitter is grounded through a bias resistor 43. A bias voltage is applied to the base by a first switching transistor 44. A parallel resonance circuit formed by a first inductor 45 and a first varactor diode 46 for resonance is connected between the base and the ground. A feedback capacitor 47 is connected between the base and the emitter, and a feedback capacitor 48 is connected between the emitter and the ground. The emitter outputs an oscillation signal.
A second oscillation circuit 51 contains a second common-collector oscillation transistor 52. The emitter is grounded through a bias resistor 53. A bias voltage is applied to the base by a second switching transistor 54. A parallel resonance circuit formed by a second inductor 55 and a second varactor diode 56 for resonance is connected between the base and the ground. A feedback capacitor 57 is connected between the base and the emitter, and a feedback capacitor 58 is connected between the emitter and the ground. The emitter outputs an oscillation signal.
A switching voltage for turning ON one of the first switching transistor 44 and the second switching transistor 55 and turning OFF the other is applied to the base of each of the first switching transistor 44 and the second switching transistor 54. Thus, one of the first oscillation circuit 41 and the second oscillation circuit 51 oscillates. The oscillation frequency can be changed by changing a control voltage applied to the cathode of each of the first and second varactor diodes 46 and 56.
In the known oscillator, the bias resistors 43 and 53 w connected to the emitters are connected in parallel to the feedback capacitors 48 and 58, respectively. An oscillation signal current is directed to flow through the bias resistors 43 and 53. As a result, the oscillation power is reduced, and the phase noise increases.
Accordingly, it is an object of the present invention to easily prevent a reduction in oscillation power, which is caused by an emitter bias resistor in an oscillation transistor, without adding a special circuit or component.
In order to achieve the foregoing objects, a dual-band oscillator according to the present invention is provided including a first oscillation circuit; a second oscillation circuit; and a switching device for alternatively operating the first oscillation circuit and the second oscillation circuit. The first oscillation circuit includes a first oscillation transistor; a first inductance element having one end grounded and the other end coupled to the base of the first oscillation transistor; and a first emitter bias resistor for setting the emitter current of the first oscillation transistor. The second oscillation circuit includes a second oscillation transistor; a second inductance element having one end grounded and the other end coupled to the base of the second oscillation transistor; and a second emitter bias resistor for setting the emitter current of the second oscillation transistor. The first emitter bias resistor is provided between the emitter of the first oscillation transistor and the second inductance element. The second emitter bias resistor is provided between the emitter of the second oscillation transistor and the first inductance element. Accordingly, a loss of oscillation signal of the operative oscillation circuit can be reduced by utilizing the inductance element in the inoperative oscillation circuit.
The switching device may include a first switching transistor and a second switching transistor. The first switching transistor and the second switching transistor may be turned ON/OFF in such a manner that one is ON while the other is OFF. A bias voltage may be applied to the base of the first oscillation transistor by the first switching transistor. A bias voltage may applied to the base of the second oscillation transistor by the second switching transistor. Accordingly, the two oscillation circuits can be switched between operative and inoperative states.
The switching device may include a first switching transistor and a second switching transistor. The first switching transistor and the second switching transistor may be turned ON/OFF in such a manner that that one is ON while the other is OFF. The first switching transistor may be provided in series to the first emitter bias resistor. The second switching transistor may be provided in series to the second emitter bias resistor. Accordingly, the emitter current of each oscillation transistor flowing into each inductance element can be interrupted by each switching transistor.
The first switching transistor may be turned ON/OFF by the second switching transistor. Accordingly, it is only necessary to apply the switching voltage for switching between ON and OFF states to the second switching transistor.
The first inductance element may be provided with a first center tap for dividing the first inductance element into two. The second inductance element may be provided with a second center tap for dividing the second inductance element into two. The first center tap may be grounded through a first capacitor. The second center tap may be grounded through a second capacitor. Accordingly, the high frequency current flowing from the emitter of each oscillation transistor to the ground can be reduced.
A parallel resonance circuit formed by one portion of the divided first inductance element and the first capacitor may be caused to resonate with the oscillation frequency of the second oscillation circuit. A parallel resonance circuit formed by one portion of the divided second inductance element and the second capacitor may be caused to resonate with the oscillation frequency of the first oscillation circuit. Accordingly, the high frequency current flowing from the emitter of each oscillation circuit to the ground can be reduced furthermore.